Obesity glossary: terms explained in plain language

Key points

33 common terms, in plain language. From adipose tissue to weight stigma, each with a link to a page with more detail.
BMI is a screening tool, not a diagnosis. Experts advise confirming excess body fat with a waist measure or a body-fat scan, except when BMI is above 40.
Waist size adds information. A waist-to-height ratio of 0.5 or more points to higher health risk, even when BMI looks healthy.
Some names have changed. Fatty liver disease linked to metabolism is now called MASLD, partly because older words felt stigmatising.
Words shape care. Person-first language, such as a person living with obesity, puts the person before the condition.

A

Adipose tissue: Body fat tissue. It stores energy and also acts as an organ that releases hormones and signalling substances. What is obesity?

B

Bariatric surgery: Surgery on the stomach and/or intestines to treat severe obesity and related conditions, such as a sleeve gastrectomy or gastric bypass. Also called metabolic surgery. Bariatric surgery
Binge eating disorder: An eating disorder in which a person regularly eats a very large amount of food in a short time while feeling a loss of control, followed by distress. It is treatable. Psychological support
Body mass index (BMI): Weight in kilograms divided by height in metres squared. For adults, a BMI of 30 or higher is classed as obesity. BMI is a screening tool, not a diagnosis. BMI and waist size

C

Clinical obesity: A term proposed by an international Lancet Commission for obesity that is already causing signs of reduced organ function or limits on daily activities. Clinical obesity
Combined lifestyle intervention: A structured programme, usually lasting months, that works on eating, physical activity, sleep and behaviour together, guided by professionals. Lifestyle treatment
Comorbidity: A condition that occurs alongside another condition, for example type 2 diabetes or high blood pressure in a person living with obesity. Health risks

D

Dumping syndrome: Symptoms such as dizziness, sweating, cramps or diarrhoea after eating, mostly after sugary food, that can occur after some types of bariatric surgery. Bariatric surgery

E

Energy balance: The balance between the energy you take in through food and drink and the energy your body uses. It is regulated by the brain and influenced by many factors. Causes

G

Gastric bypass: A type of bariatric surgery in which a small stomach pouch is created and connected directly to the small intestine, bypassing most of the stomach. Bariatric surgery
Ghrelin: A hormone made mainly in the stomach that increases hunger. Levels often rise after weight loss. Genetics and biology
GLP-1: Glucagon-like peptide-1, a gut hormone that increases fullness, slows stomach emptying and helps control blood sugar. Several weight-loss medicines mimic it. Weight-loss medication

H

HbA1c: A blood test that shows your average blood sugar over the past two to three months. It is used to diagnose and monitor diabetes. Doctor visit checklist

I

Insulin resistance: A state in which the body responds less well to insulin, so more insulin is needed to keep blood sugar normal. It can lead to type 2 diabetes. Health risks

L

Leptin: A hormone made by fat tissue that tells the brain how much energy is stored. Rare genetic defects in leptin cause severe obesity from early childhood. Genetics and biology

M

MASLD: Metabolic dysfunction-associated steatotic liver disease: a build-up of fat in the liver linked to metabolic risk factors. It was previously called NAFLD. Health risks
Metabolic adaptation: The body's response to weight loss: it uses less energy and increases hunger signals, which makes keeping weight off harder. Weight regain

O

Obesity: A chronic, complex disease defined by excessive fat deposits that can impair health. In adults it is usually screened for with a BMI of 30 or higher. What is obesity?
Obesogenic environment: Surroundings that make weight gain more likely, for example easy access to cheap, energy-dense food, large portions, constant food marketing and little need to move. Food environment
Obstructive sleep apnoea: A condition in which the airway repeatedly narrows or closes during sleep, causing breathing pauses, poor sleep and daytime tiredness. Health risks
Overweight: In adults, a BMI of 25 to 29.9. In children, growth charts for age and sex are used instead. BMI and waist size

P

Person-first language: Language that puts the person before the condition, such as 'a person living with obesity' instead of 'an obese person'. Weight stigma
Polycystic ovary syndrome (PCOS): A common hormonal condition that can cause irregular periods, reduced fertility and weight gain, and is linked to insulin resistance. Health risks
Preclinical obesity: A term proposed by an international Lancet Commission for excess body fat that increases health risks but does not yet cause signs of illness or limits on daily life. Clinical obesity

S

Semaglutide: A medicine that mimics the hormone GLP-1. It is prescribed by a doctor, as a weekly injection or in some countries a daily tablet, and is approved in many countries for obesity. Common side effects include nausea, and weight usually returns after stopping. Weight-loss medication
Sleeve gastrectomy: A type of bariatric surgery in which a large part of the stomach is removed, leaving a narrow tube-shaped stomach. Bariatric surgery

T

Tirzepatide: A medicine that acts on two gut hormone receptors, GIP and GLP-1. It is prescribed by a doctor as a weekly injection and is approved in many countries for obesity. Common side effects include nausea, and weight usually returns after stopping. Weight-loss medication
Type 2 diabetes: A chronic condition in which blood sugar is too high because the body does not respond well to insulin or does not make enough. Obesity is a major risk factor. Health risks

U

Ultra-processed food: Industrially made food and drink products with ingredients rarely used in home cooking, such as additives, flavourings and modified starches. Food environment

V

Visceral fat: Fat stored deep in the abdomen around organs. It is more strongly linked to health risks than fat under the skin, which is why waist size matters. BMI and waist size

W

Waist-to-height ratio: Your waist circumference divided by your height. A ratio of 0.5 or higher suggests increased health risk from abdominal fat. BMI and waist size
Weight bias: Negative attitudes and assumptions about people because of their body weight, which can be conscious or unconscious. Weight stigma
Weight stigma: The social devaluation of people because of their weight, shown in stereotypes, unfair treatment and discrimination. It harms physical and mental health. Weight stigma

Which terms have changed, and why?

Medical words change as knowledge grows and as patients are heard. The table shows four changes you may notice in letters, reports or the news.

Older terms and the terms used now
Older term	Term used now	Why it changed
an obese person	a person living with obesity	Puts the person first and reduces stigma
NAFLD (non-alcoholic fatty liver disease)	MASLD	The words non-alcoholic and fatty felt stigmatising to 61 % and 66 % of the experts and patient advocates consulted
weight-loss surgery	metabolic and bariatric surgery	The operations also improve or put into remission conditions such as type 2 diabetes
obesity, judged by BMI alone	clinical or preclinical obesity	BMI cannot show whether extra body fat is already harming organs or daily life

Which numbers will you hear about at the doctor?

Four numbers come up often: BMI and waist size, the waist-to-height ratio and HbA1c, a blood sugar test. Each has its own entry above.

No single number tells the whole story. Your doctor looks at them together with how you feel and how your body works.

Where can you read more about these terms?

Start with what obesity is, then read about its causes and treatment options. Questions for your doctor are on the doctor visit checklist.

Frequently asked questions

Why do doctors use BMI if it is not a diagnosis?

BMI is quick, cheap and needs only weight and height, so it works well as a first check. An international Lancet Commission advises confirming excess body fat with a waist measure or a body-fat scan, unless BMI is above 40. A diagnosis also looks at whether body fat already affects health.

Why was fatty liver disease renamed MASLD?

Liver experts and patient groups chose a new name because the old one relied on what the disease is not and used words many people found stigmatising. MASLD stands for metabolic dysfunction-associated steatotic liver disease, and it requires at least one metabolic risk factor.

What is the difference between weight bias and weight stigma?

Weight bias is the attitude: negative beliefs about people because of their weight, often unconscious. Weight stigma is what follows from it, such as jokes, unfair treatment or poorer care. Both harm health.

What does ultra-processed mean?

Ultra-processed foods are industrial products made mostly from substances extracted from foods, often with additives such as flavourings, colours, sweeteners or emulsifiers. A practical check is the ingredient list: if it contains items you would not use in a home kitchen, the product is probably ultra-processed.

What does metabolic and bariatric surgery mean?

It is the current name for operations on the stomach or bowel to treat severe obesity. The word metabolic shows that the surgery can also improve, or put into remission, related conditions such as type 2 diabetes. It always needs lifelong follow-up.

Obesity glossary